The present invention relates to a thermal-type air flow measuring instrument for measuring air flow taken into the combustion engine for automotive vehicles and measurement-error correcting apparatus.
In terms of environmental protection and resource saving, more accurate combustion control is required for the engines of automotive vehicles, and air flow meters capable of detecting inlet air flow with high accuracy are desired. Such control systems as those employing thermal-type air flow measuring instruments capable of measuring directly air mass flow are mainstream today.
One of such measuring instruments is disclosed in Japanese Patent Application Laid-Open No. 8-278178 (1996). The measuring instrument is constructed so as to cancel a temperature characteristic of the thermal-type flow measuring instrument with a temperature characteristic of an electronic circuit. In more detail, the thermal-type flow measuring instrument disclosed therein corrects a dependence of a measurement error depending upon variation of temperature, relative to an air flow rate, by providing a flow rate dependence to cancel the measurement error for the temperature characteristics of a reference voltage generating circuit incorporated in the thermal-type flow measuring instrument.
Another conventional measuring instrument is found in Japanese Patent Laid-Open No. 60-100218. This prior art discloses and teaches the correction method of temperature characteristic of the thermal-type air flow measuring instrument.
The former cancels the temperature-error contained in an output of a bridge circuit formed with a heating resistor, a temperature-sensitive resistor or the like to be caused depending upon temperature variation of the fluid, corresponding to the flow rate, by a temperature characteristic of the electronic circuit. Accordingly, a problem is encountered in that when a difference is caused between a temperature of the fluid and the temperature of the electronic circuit, the correction becomes in effective.
In the latter, the correction of the temperature characteristic is established by adjusting the flow signal only with a definite voltage value in response to the intake air temperature without considering the measured air flow. In practical conditions, however, the output signal corresponding to the air flow is subject to the temperature of air flowing in the heating resistor. This is caused by the following reasons.
The thermal-type air flow measuring instrument is composed of a bridge circuit, in which the output voltage Vout is defined by the formula 1. EQU Vout=.multidot.(A +B(Q)) (1)
In the formula 1, constants A and B are constant with respect to the air flow Q, but have temperature sensitive characteristics. This is because temperature sensitive characteristics of constants A and B are influenced by the thermal conductivity of air. In other words, the constants A and B reflect changes in physical properties of air such as thermal conductivity and kinematic viscosity. Thus, the differential coefficient of the output of the bridge circuit with respect to the temperature T, that is, dV/dT, is dependent upon air flow. In addition, the influence of thermal conduction from the heating resistor to the members on which the heating resistor is supported is also another factor of air flow dependency. Thus, in the prior art, there is such a problem that correction of temperature characteristic can not be established for the wider range of air flow and for the wider range of working temperature.